1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of uniformly forming a thin film on the surface of a semiconductor substrate.
2. Description of the Prior Art
An effort for further enhancing refinement and high density of the structure of semiconductor devices such as insulated gate field effect transistor (referred to as MOS) is still being pushed vigorously at this date. Such a refinement, being the most effective technique for imparting them with high performance or multifunctional capability through higher degree of integration, higher processing speed and the like of semiconductor devices, is becoming an indispensable process for the manufacture of future semiconductor devices.
At present, 256 megabit DRAM products have been developed on 0.13 xcexcm design standard and are being manufactured. Moreover, in addition to such memory devices, development of logic devices with refined structure or a variety of mixed devices having fine structure, such as mixed logic memory devices, analog mixed logic devices, or the like, is presently under examination.
In conjunction with endowing high performance and multifunctional capabilities with the semiconductor devices mentioned above, it is necessary, along with the refinement of the design dimensions of the semiconductor devices, to enhance the quality of insulator films, conductor films or semiconductor films that constitute the semiconductor devices. In particular, reduction in the thickness and increase in the quality of the gate insulating film of a MOS transistor or the capacity insulating film of a capacitor are deemed .indispensable technological objectives.
In the conventional formation of a silicon oxide film which serves as the gate insulating film, it has been general to employ a method in which an oxidizing gas such as oxygen (O2) gas or steam (H2O) gas, as in pyrogenic oxidation, is used. In the formation of an oxynitride film, use of nitrous oxide (NO) gas or ammonia (NH3) gas is general. However, thin films formed by the use of such a reaction gas is confronting the limit for improving the quality in view of their application to semiconductor devices.
Under these circumstances, various methods for forming a silicon oxide film or an oxynitride film to be used as the thin film for the gate insulating film by means of the reaction between an active species obtained by activating an oxidizing gas or a nitriding gas and a silicon substrate, have been under investigation. Such active species include, for example, oxygen or nitrogen that is converted to neutral free radical. At this point, as a technology readily applicable to mass production of a semiconductor device, there may be mentioned, for example, a method of forming a silicon oxide film by the use of an oxidizing active species obtained by the in-situ steam generation (ISSG) method as disclosed in IEEE Electron Device Letters, Vol. 21, No. 9, September 2000, pp. 382-384. In addition, as a technology for forming a silicon oxide film or an oxynitride film by the use of an active species such as an oxygen or nitrogen free radical, one might mention, for example, the methods disclosed in Japanese Patent Applications Laid Open, No. 2000-286259 and Japanese Patent Applications Laid Open, No. 2001-291866.
Referring to FIG. 10 to FIG. 12, formation of a silicon oxide film by the ISSG method will now be described as a representative example of the conventional technology.
FIG. 10 is a schematic sectional view of a reaction chamber section of a single wafer type film formation device. In a reaction chamber section 101, a rotating support substrate 103 with a wafer 104 placed thereon, is disposed below a reaction chamber 102. Above the reaction chamber 102, there is provided a lamp chamber 106 via a transparent glass window 105. The wafer 104 is heated to about 1000xc2x0 C. by the lamp. Oxygen gas and hydrogen (H2) gas are introduced separately into the reaction chamber through a gas introducing port 107, and the gas after the reaction is discharged to the outside by a pump through a gas discharge port 108. The gas pressure inside the reaction chamber is arranged to be reducible to below 2xc3x97103 Pa. The reaction chamber 102 is connected to a load lock chamber (not shown), and the wafer 104 is loaded/unloaded through the load lock chamber.
A feature of the ISSG method resides in the point that the reaction chamber is kept at a reduced pressure, differing from the conventional oxidation method. Oxygen and hydrogen entering the reaction chamber through the gas introducing port 107 are subjected to a thermal reaction under the reduced pressure to be converted to steam (H2O), oxygen atom (O) and a hydroxyl free radical (OH), brought into reaction with the surface portion or the like of the silicon substrate being the wafer 104, and forms a silicon oxide film on the wafer 104.
Referring to FIG. 11, the sequence of processes of the ISSG method in the conventional film formation method will be described. In FIG. 11, the abscissa shows the processing time and the ordinate shows the gas pressure in the reaction chamber 102. As shown in FIG. 11, prior to the film formation, nitrogen gas is introduced into the reaction chamber 102 to be kept at a prescribed reduced pressure state (for example, at a pressure of 2xc3x97103 Pa). Then, the wafer 104 is brought into the reaction chamber from the load lock chamber. Following that, the pressure inside the reaction chamber is reduced to, for example, about 7xc3x97102 Pa, oxygen gas and hydrogen gas are introduced into the reaction chamber 102 as shown in FIG. 11, and the formation of the silicon oxide film is started. During the film formation, the gas pressure in the reaction chamber 102 is kept constant (at 7xc3x97102 Pa in the above example). The wafer 104 is rotated during the process in order to enhance the uniformity of the in-plane thickness of the wafer. Upon completion of the film formation the introduction of oxygen gas and hydrogen gas is stopped, they are replaced by nitrogen gas and its pressure is returned to the value (2xc3x97103 Pa in this example) before the above treatment. Thereafter the wafer 104 is taken out to the load lock chamber.
Next, the film thickness distribution will be described with reference to FIG. 12 when a silicon oxide film is formed on the surface of the silicon substrate according to the above process sequence. Here, the abscissa shows the location in the radial direction from the center of the wafer. In this example, the wafer is a silicon wafer of 8xe2x80x3 diameter, and the wafer position X represents the distance of the point under consideration measured from the center of the wafer as indicated by the percentage relative to the radius of the wafer. The ordinate shows the thickness of the silicon oxide film. As shown in FIG. 12, the distribution is such that the thickness of the silicon oxide film increases from the central part of the wafer toward its outer periphery, reaches a maximum past midpoint, then decreases toward as one moves toward the outer periphery of the wafer. The dispersion of the thickness in the wafer plane of the silicon oxide film is about +/2%. There is no change in the film thickness in the rotational direction of the wafer.
With the enhancement of the performance of the semiconductor devices, it is general to demand prevention of deterioration in the quality of the formed thin film and to reduce the dispersion of the film thickness, along with the requirement for high precision of the processing dimensions. Moreover, since the increase in the wafer diameter is the most effective means for enhancing the yield of nondefective products, the shift on the mass production line from the 8xe2x80x3 silicon wafer to a larger diameter wafer of 12xe2x80x3 wafer is destined to take place. Under these circumstances, further reduction in the dispersion of the film thickness of the thin films becomes necessary when thin films such as silicon oxide film are to be formed by means of the active species such as done in the conventional technology described in the above. Considering the requirement for high performance and the increase in the diameter of the wafer for the future semiconductor devices, it is indispensable to suppress the dispersion of the film thickness to below +/1%. However, according to the result of various kinds of investigation carried out so far by the present inventor, indication is that the dispersion of the film thickness of the silicon oxide film by the present state of art of the ISSG method is +/2% at the best. With this knowledge in mind, the present inventor searched and experimented various methods toward further enhancement of the dispersion of film thickness in the wafer plane when the film is formed generally by the use of active species such as neutral free radicals.
As a result of the search, it was found when a film is formed by the use of active species, without being limited to the case of the ISSG method, that the dispersion of the film thickness depends very heavily on the change with the lapse of time of the active species in the reaction chamber for film formation. The present invention is based on such new findings by the present inventor.
Summary of the Invention
This is a method of manufacturing a semiconductor device in which a thin film is formed on a semiconductor substrate by using reaction gas of the present invention as active species, wherein a film is formed by subdividing the film formation process into multiple stages, and the gas pressure or the gas flow speed of the active species is varied in each stage.